The green liquor in the causticizing process consists mainly of sodium carbonate. The green liquor in the process reacted with quick lime wherein the lime effectively reacts exothermically with a part of the water in the liquor and forms slack lime, calcium hydroxide. The slack lime reacts with the sodium carbonate in the green liquor and forms calcium carbonate, lime sludge, and sodium hydroxide, white liquor. The white liquor is separated out of the causticizing process and is reused in sulphate process for cooking paper wood chips, and the lime sludge is separated out of the process and is reused by burning to slack lime.
In a causticizing process according to known technique, the separation of white liquor and lime sludge takes place in tube filters, i.e. in the suspension immersed perforated tubes covered with filter cloth through which the white liquor is pressed. The dry content of the effluent lime sludge is about 30-35%. The lime sludge is washed in a subsequent wash step by dilution with water and subsequent thickening in a tube filter. The lime sludge dry content obtained herewith is also comparatively low, about 35%. Finally, the lime sludge suspension is filtered and the lime sludge is washed with constriction on a vacuum filter and a lime sludge dry content of about 70% is obtained.
Tube filters for separating lime sludge and white liquor accordingly yield a relatively low lime sludge dry content, about 30-35%. This means a low efficiency and a load on subsequent lime sludge wash. Moreover, tube filters are usually not operating continuously. The filtering operation must be interrupted with about 4 minute intervals for removing the filter cake on the filter cloth and cleaning the filter cloth by backwashing. Such a method means a complication in a process which otherwise is continuous.
According to known technique vacuum filters also are used in causticizing processes. Vacuum filters are, however, burdened with disadvantages. They must operate with high vacuum in order to yield a good wash effect and high outgoing mass dry content. Required vacuum sets limits to the temperature of the suspension to be filtered and which seldom may exceed 70.degree. C. In causticizing processes known equipment for the separation of lime sludge and a white liquor and for the removal of white liquor out of the process, i.e. tube filters and vacuum filters, are accordingly badly adapted for effective filtering under pressure and at high temperature. Neither can washing of the lime sludge in tube filters be effectively accomplished; and the vacuum filters can only be used with a limited difference pressure and cause large heat losses.
For separating sludge from green liquor in present causticizing processes, green liquor clarifiers are used. A green liquor clarifier requires huge space and large investment costs as well as huge costs in connection with emptying the clarifier for internal maintenance thereof. Moreover, sludge with low dry content is obtained.